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Seating Orthosis Design for Prevention of Decubitus Ulcers

J. Martin Carison, MS, CPO
Mark J. Payette
Lisa P Vervena, MS

ABSTRACT

Introduction

Many diagnoses severely limit or end ambulation for people who are still in their youth or middle years. Of those diagnoses, neuromuscular conditions that destroy or seriously impair skin sensation are most likely to lead to skin breakdown. For purposes of technical focus, this article will relate primarily to people with spinal-cord injury, but in nearly all respects, the principles presented are generally applicable to clients with a combination of motor and sensory loss.

Decubitus ulcers are lesions ranging from redness of the skin (nonbianchable erythema/stage I) to tunneling ulcers with necrosis of the skin, fat, muscle and bone (stage IV). The term "decubitus ulcer" is used in preference to "pressure sore" or "pressure ulcer" because pressure is only one of many factors leading to formation of the lesion.

The morbidity and costs directly related to treatment of decubitus ulcers are difficult to estimate but are known to be quite substantial (1). Skin breakdown creates a portal for infectious invasion, and that portal is virtually impossible to defend over time. Decubitus ulcers often lead to hospitalization, plastic surgery and amputation. The medical histories of the majority of clients with decubitus ulcers who were referred to Tamarack Habilitation Technologies for custom orthotic seating intervention indicate related medical costs estimated between $50,000 and $500,000. In addition to the direct medical and health consequences, decubitus ulcers often devastate careers, upend lifestyles, reduce independence and lead to depression.

It is fairly common for young adults to be able to sit safely on one of the standard (pneumatic or jelly envelope) cushions for five to 10 years after their injury. However, the authors have observed that, without functional paraspinal and abdominal musculature, posture and spine alignment slowly degrade with time. The pelvis tilts posteriorly during sitting, and the thoracolumbar spine collapses into hyperkyphosis.

In some cases, the spinal collapse includes scoliosis. (The authors have encountered scoliotic collapse more frequently among clients with thoraciclevel lesions than among those with lesions at cervical levels.) These postural changes elevate the two major risk factors, as discussed later. In addition, all physiologic systems, including skin and circulation, gradually become less resilient and viable with time and the natural aging process. The margin of safe sitting function becomes narrower year by year, and the likelihood of something triggering a skin breakdown increases.

Even after a decubitus ulcer has healed, the skin does not fully recover its pre-ulcer margin of safe function. Scarring, adhesions and tissue loss in the wake of a decubitus ulcer significantly increase future risk. Each subsequent ulceration further elevates the hazard. Re-ulceration following surgical closure typically occurs along the closure scar (see Figure 1 ). Prevention of re-ulceration often requires different seating technology and/or an alteration of lifestyle even when the breakdown occurred as a result of a unique event or circumstance that can be avoided in the future. If the re-ulceration cycle cannot be prevented, the person's life recedes further and further from the possibility of returning to normal, and immense personal devastation becomes increasingly certain. Intervention must occur as early and strongly as possible to prevent spiraling costs and suffering.

For readers who may not have much experience with designing and providing systems for prevention and healing of decubitus ulcers, it may be helpful to review a rather typical case history:

L.W. was referred for custom orthotic seating services at the age of 35. He lives with his wife, who takes an active helping and monitoring role. His cervical-level traumatic spinal cord injury occurred when he was 20. Following his initial rehabilitation, he had been discharged from the rehabilitation facility with a rather simple, inexpensive foam cushion that seemed adequate for his needs at that time.

L.W. developed his first ulcer five years after his spinal cord injury and eventually had plastic surgery to close it. At that time, he changed to a pneumatic cushion. He was then free of decubitus ulcers for about seven years. During that period, L.W was employed full-time.

During the three years immediately preceding referral for custom seating services, L.W. had multiple decubitus ulcer incidents and two additional plastic surgeries. At the time of his referral, he had an open ulcer at one of his ischial tuberosities measuring 10 cm x 6 cm and 2to 3-cm deep. L.W. had been under doctor's orders not to sit for virtually all of the previous year. He had lost his full-time job. He had recently decided he could not continue his life from a prone position so he would sit up and function as best and as long as he could, no matter what the consequences. As a result, his decubitus ulcer was growing in size, and his prognosis, given his circumstances and decision, was poor.

A custom-seating orthosis for management of shear and pressure was designed, fabricated and fit L.W. continued to sit an average of 14 hours per day. After approximately seven months of using the orthosis, his wound had closed, and he was on a waiting list to get his job back.

Decubitus Ulcer Generation Factors

Most decubitus ulcers form over weightbearing bony prominences. In seating, the most frequently involved areas are over the sacrum, coccyx. ischial tuberosities and greater trochanters. Four local factors contributing to the generation of decubitus ulcers are generally recognized: pressure, shear, temperature and moisture. These factors are labeled "loca1" because there are also many contributing systemic factors (e.g., vascular health, muscle tone, nutrition, age, etc.) and global factors (client education. motivation, lifestyle, program follow-up, etc.) (2,3).

Pressure

The factor which has been assumed primary for many years is simple pressure, acting roughly perpendicular to the skin surface. The popular interpretation of physiologic events is that when the pressure on tissues exceeds blood capillary pressure, blood cannot carry nutrients to the tissue cells. Also, of course, waste products cannot be transported away from the cells.

It is fairly easy to understand that if excessive pressure deprives tissue cells of nutrients long enough, they exhaust available nutrition and begin to die. Any relief of the offending pressure before necrosis has proceeded will allow nutrients back into the area and waste products to be carried off, giving the tissues a new lease on life. The length of that "new lease" depends upon how much blood transport has occurred during the pressure relief. In fact, if the pressure variations are frequent enough, they can act as a pump, actually aiding circulation.

For those who have sustained a spinal cord injury or are elderly or ill, the blood flow may already be compromised. Bennett et al. (4,5) have found that the median blood volume flow rate of these subjects is only about one-third that of healthy subjects. (For active people without neuromuscular impairment, muscle activity, in conjunction with venous valves, functions as a venous return pumping system.)

The figure popularly used to represent blood capillary pressure is 32 mmHg (6,7) although some measurements of the functional mean capillary pressure are as low as 17 mmHg (8). Thirty-two mmHg is a very low pressure compared with the level of most physiological pressures encountered in everyday life. As we walk, twist the cap off a bottle or use a pair of pliers, we generate skin surface pressures greater by more than an order of magnitude. Those pressures are not harmful under normal circumstances because they are of short duration and/or not repetitive.

Pressure is calculated in units of force per unit of area. The greater the area over which a given force is applied, the lower the pressure. So pressures under the foot while standing and walking are greater than sitting surface pressures. Lindan et al. (9) measured pressures as high as 100 mmHg under seated people. Recumbent support pressures, because of the larger contact area, were measured only as high as 60 mmHg (9). The total support area, however, is only part (sometimes a minor part) of what determines the magnitude of pressures at bony prominences. Pressures vary over the support surface depending upon how close the skeleton is to the skin surface. Sitting pressure measurements vary significantly more for paraplegic and elderly people than for neuromuscularly healthy people even though median pressure values are similar (5). This may be due to generally lower muscle tone and widely variable amounts of fatty "padding."

Asymmetrical sitting posture, spine collapse or both, which are commonly observed clinically, profoundly affect decubitus ulcer risk by altering the location and magnitude of peak pressures. A habitual leftward trunk alignment, for instance, shifts trunk weightbearing forces and concentrates them under the left ischium. Scoliotic spine collapse almost always includes lateral tilt of the pelvis, which also concentrates pressure under one isehium (10). So maintaining isehial pressures at safe levels is directly and profoundly related to providing postural alignment control and orthopedic spine support.

Body build also affects the pressure distribution. It is more likely that thin people have a high pressure area over a bony prominence than people of average or above-average weight (11). A reduced level of compliance (or "softness") at isehial tuberosities, for instance, causes those areas to bear a greater portion of the load. When the hamstring muscles are tensed or in spasm, they are less compliant, and those posterior thigh tissues will then bear a greater portion of the total weightbearing load. So hamstring muscle activity will cause load to be shifted away from other weightbearing areas, especially the isehial tuberosities. These points are easily verified using a simple skin pressure evaluator.

Applying pressure to the skin has some secondary effects as well. If the pressure is high enough, some edema results when pressure is relieved (12). This swelling increases the distance between capillaries and tissue cells, and decreases the rate of transport of nutrients to, and waste products from, those cells (12,13). In addition, the local skin temperature may rise as much as 1.9°C (3.4°F) after pressure is relieved and only gradually decrease back to normal (14). The effect of such a temperature increase is detrimental and will be discussed later.

Shear

When one talks about pressures related to standing, walking, sitting or lying, one is generally referring to pressures generated by support surfaces acting roughly perpendicular to the contact surface. Friction is the second important force component present in most cases. Friction forces act parallel (or tangential) to the skin surface and produce shear strains within the skin and underlying tissue. Frictional forces and their effects are present whenever there is either sliding or a tendency to slide.

Shear strains (tissue motions divided by the thickness across which they occur) are higher where tissue between skin and bone is thin. Shear strain stretches and tears microstructures such as cell walls and capillaries. (Scissors are the ultimate shearing tool and are, in fact, often called shears.") Shear stresses (frictional force divided by area) concentrate in areas that have scarred-down, adhered or "tethered" to underlying bone because these factors affect tissue shear compliance. This is probably one of the leading reasons why, with each occurrence, it is increasingly difficult to prevent recurrence of decubitus ulcers. It is important to note that friction-induced stresses will not exceed a certain fraction (depending on the nature and material of the particular surfaces involved) of the contact pressure. Friction forces and the tissue shear they cause will therefore be limited in areas of lesser contact pressure.

Shear stress is difficult to measure, and the mechanics of tissue destruction by shear are not easy to model. Because good research experiments and clinically relevant models are scarce, the shear factor unfortunately has been largely ignored in clinical practice and product design. However, it is known that when shear is measured under the buttocks of subjects seated on a hard seat, the results are almost three times higher for people who are elderly, ill or paraplegic than for people without neuromuscular impairment (4,5). The pressure and magnitude of shear is very dependent upon the posture of the sitter as well as the spatial orientation of the support surfaces.

Figure 2a , Figure 2 , Figure 2c , Figure 2d , and Figure 2e provide very strong evidence, albeit anecdotal, of the significance of shear as a factor in skin breakdown. The man whose buttocks are shown in these photographs suffered a severe injury that resulted in the destruction of his right hip and knee joints. The skin was stripped from a very large area of his right pelvis, hip and buttocks. His right hip was fused in about 40 degrees of fiexion while his right knee was fused at full extension. He had suffered repeated skin breakdowns over the area of grafted skin.

Figure 2a shows the large areas of skin breakdown present at the time he was referred for a seating consultation. He was observed transferring into and out of his seat. His fused right hip seemed to cause a high magnitude of shear and much dependence on friction to maintain his position on the pneumatic seat cushion in his wheelchair.

The only intervention provided for this gentleman was to adjust and reinforce his wheelchair footrest so that, as he settled into his seat, he was stopped from sliding by the elevated footrest bearing against the bottom of his foot, not by friction on the sitting surface. He had protective sensation in his leg and foot and was able to adjust his position and the pressure on his foot as necessary throughout the day. Absolutely no changes were made to this man's sitting support surface. Figure 2b , Figure 2c , Figure 2d , and Figure 2e show his steady healing progression during the following months. Common sense would lead us to suspect that a combination of pressure and shear would be more dangerous than either factor alone. Several researchers have verified that less pressure will cause the same blood flow obstruction when shear is also acting on the tissue (4,15,16).

Temperature

Temperature is a factor in decubitus ulcer formation because of the relationship between temperature and metabolic rate. The common rule of thumb says a 1°C (1.8°F) elevation in the temperature of a human tissue cell will increase the metabolic rate by approximately 10 percent (13). Body temperatures may increase due to systemic responses to many factors. Local increases in tissue temperature can be caused by the hyperemia that is the body's natural healing response to slight amounts of tissue damage.

Also, local tissue temperatures are certainly affected by the heat transfer properties of the sitting support surface materials. Ferguson-Pell et al. found temperature elevations of 3.75°C (6.75°F) after sitting on certain wheelchair cushions for two hours (17), which translates to a 37 percent higher metabolic rate. Tissue necrosis would therefore begin in 37 percent less time when pressures are high enough to limit adequate transport of nutrition to the tissue cells. This helps explain how a small amount of trauma resulting in a bit of hyperemic temperature rise can destabilize a previously safe, but marginal, condition.

Moisture

Moisture is both a direct and an indirect factor in decubitus ulcer generation. The effect on skin condition depends upon the nature of the moisture itself-whether the moisture is from urine, bowel material, perspiration or a fixture of those. Ferguson-Pell et al. found a 15 percent to 39 percent increase in relative humidity at the seating surface after just two hours of sitting (17). From the standpoint of custom orthotic seating design, it is probably sufficient to observe that sitting surface materials that have some absorbency and promote air circulation will not only help to reduce moisture at the skin surface, but they also reduce temperature by allowing evaporation of that moisture. Moisture is an indirect factor because it generally increases interface friction coefficients and, therefore, the possibility for shear-induced tissue damage.

Up to this point, the focus of the discussion has been on the nature and mechanism of various factors playing a role in skin breakdown. Although there are many contributing factors in addition to those listed above, decubitus ulcers are usually preventable with proper care and/or equipment that addresses those ulcer generation factors. Simply changing position every few hours while lying in bed, or every 15 minutes while seated, significantly reduces the risk (18). Similarly, proper lifting and transferring reduce, and may even eliminate, shear (18). In some situations, however, custom orthotic seat design may be the most reliable means to reduce the risk of decubitus ulcers.

Equipment Design

Seating design elements useful for managing tissue pressure and shear are:

• seat bottom cushioning
• seat bottom contouring
• postural alignment support
• spatial orientation of support surfaces
• use of a spinal orthosis in conjunction with custom-designed seating when significant spine collapse has occurred
• choice of interface materials

Cushioning

Cushioning materials are commonly added to a seat bottom in an attempt to reduce contact pressure. However, a wheelchair cushion that does not reflect human anatomy in its surface contouring will always result in maximum pressure on the high-risk areas, no matter how soft or thick or fluid that cushion may be. Deeper cushioning will increase the area over which the force is applied, therefore reducing the magnitude of peak pressures. However, peak pressures will continue to be present at the same locations (see Figures 3a-b ).

More cushioning is not always better. For example, we know from experience that some youngsters with cerebral palsy can sit comfortably and safely on a very firm surface when that surface is appropriately contoured (19). This is because of the high muscle tone in their posterior thigh muscles. Even slight variations of tone or flickers of spasms will, with a hard surface contour, effect large corresponding changes in the distribution of surface contact pressures. That involuntary muscle action produces less pressure variation and relief when the sitting surface is more cushioned.

Likewise, small sitting alignment variations and postural adjustments, which are the most some people can achieve, will translate to greater pressure changes when they are seated on a harder surface. So firmness of the sitting support surface is a definite benefit in some cases. Cushioning materials placed over the contoured surface reduce this beneficial effect from small motions. However, total elimination of cushioning materials can be advocated only in seating for children with high muscle tone and then only in conjunction with appropriate contouring.

Seat Bottom Contouring

An orthotist's primary means of managing pressure in any orthosis is by proper contouring of the contact surfaces (see Figure 4 ), which begins with an awareness of the skeletal anatomy. This is not a new concept for orthotists and prosthetists, and at least one method of seating orthosis fabrication has been well documented (20).

The process is begun with a landmarked impression of the sitting support surface, including the lumbar spine. The impression includes the thoracic spine if the client is quadriplegic or scoliotic. Bony landmarks must be well defined. From that impression a pattern and shell are generated, which have the proper reliefs and other contour features necessary to reduce pressure on the less compliant bony areas and transfer those loads to areas such as the posterior thighs, which have greater compliance and tolerance to pressure.

The most obvious feature affecting this redistribution is a shaped recess under the pelvis (see Figure 5 ). Particular attention must be given to lateral pelvic tilt when the client has a scoliotic spine collapse. The position and relative elevation of the isehial areas of the seat should reflect the degree of uncorrectable scoliosis that will remain after the prescribed level of orthopedic spine support has been provided. The pelvic area of the seat for a client with significant structural scoliosis should, therefore, have a recess (or recesses) for the ischia that is angled downward and to the right or left, depending on the scoliosis convexity.

The contouring also must reflect the level of certainty as to accuracy and repeatability of the client's position in the seat. As the final contour and combination of materials is approached, surface pressure measurements should be used to guide the orthotist to the final configuration (see Figures 6a-b ). These measurements should assure that pressures on at-risk areas are at tolerable levels (less than 25 mmHg).

The magnitude of pressure reduction at high-risk locations depends on the depth of the corresponding recesses. If desired, deep contouring can eliminate contact altogether. The authors do not feel such an elimination of all contact is necessary or wise except for posterior pelvic/sacral surfaces. Under isehial decubiti, for instance, the authors endeavor to maintain contact but at the very low levels cited above.

Another useful technique for reducing pelvic area contact pressures is to use lower-leg weight acting across a proximal thigh fulcrum to reduce the weightbearing under the pelvis. This is accomplished by contouring the seat bottom to produce a fulcrum on the thigh support surface between the ischial tuberosities and the mid-thighs (see Figure 7 ).

The benefits, of course, are not realized without proper positioning of the user's footrests. The footrests must be dropped down to the point where the footrests bear only a minor portion of the lower-leg weight. This can be an extremely effective way to reduce isehial contact pressures. With proper seat bottom contouring and lowering of the user's footrests, trials by the primary author show the fulcrum effect can reduce isehial pressure readings from 70 mmHg to 20 mmHg. It is fortuitous that the production of this "thigh rocker" contour fits well with the pelvic recess mentioned earlier and the "cradling" concept to be discussed later. When able-bodied people "sit on their hands," they are making intuitive use of the thigh fulcrum effect.

The overlayer of cushioning materials placed on top of the base contour of a custom contoured seat, as well as some of the standard production designs, accommodates imprecision in the patient's placement on the seat. Also, for the higher-functioning user, some necessary variation of position may occur as the person goes about daily activities. These variations must be accommodated by both the base contouring and the cushioning layer.

Postural Support

A postural support system that maintains the trunk as nearly vertical as practical and effectively resists kyphoscoliotic collapse has a profound effect on management of both pressure and shear. The more erect posture moves the upper body weight anteriorly and makes it possible to bear that weight on the more pressure-tolerant areas of the proximal thighs. Lateral thoracic support is often necessary to control lateral lean. More than one support may also be used in asymmetrical combination to help resist scoliotic spine collapse. Lumbar support is essential to resist kyphotic collapse but must be used in conjunction with a pelvic recess in the seat bottom. Otherwise, the pelvis is simply pushed forward.

Spatial Orientation of Support Surfaces

If the postural support is combined with a seat bottom that is contoured to include a pelvic recess and a thigh support inclined slightly to match the angle of seat back recline, we actually create a sitting surface that cradles the body and eliminates virtually all tendency to slide (see Figure 8 ). Contrast this with the common situation where there is a horizontal seat bottom surface and no deliberate lumbar support (see Figure 9 ). In some cases, such as with a pneumatic cushion, the seat bottom support surface may actually be declined (downward), and friction keeps the person in the chair. However, lumbar support cannot be a really effective postural support unless the seat bottom also is designed properly, as in Figure 8 . Without the pelvic recess, lumbar bolstering will simply displace the pelvis anteriorly.

Some wheelchairs with reclining backs have special provisions to eliminate friction forces on the wheelchair user's back. The seat back is on a slide mechanism. At first glance, that may appear to be a great benefit. However, for an individual who is partially reclined and sitting with a friction-free back support surface, all of the friction and shear necessary to keep that individual from sliding out of the wheelchair are now concentrated on the seat bottom areas of the user's anatomy. This certainly elevates the risk of decubitus ulcers forming over the isehial tuberosities and sacrum. Individuals who need to recline while in their wheelchairs are usually much better served by a tilt-in-space type of recline option.

Spinal Orthosis

The loss of spine and trunk muscle function typical for people with spinal cord injury causes gradual kyphotic (and sometimes scoliotic) collapse of spine/trunk alignment. The seat shell is designed to resist the spine collapse. However, effective long-term bracing of a collapsing spine is difficult at best. Therefore, in addition to the bracing action provided by the seat shell, it is almost always wise to use a fabric abdominal-use jacket type of spinal orthosis to provide additional spine support (see Figure 10 ).The fabric abdominal jacket can be a safe, functional way to restore some of the lost spine stability. The jacket substitutes for nonfunctional abdominal muscles by providing circumferential constraint. This allows the weight of the upper torso, head and arms to be borne partially by the hydraulic cylinder created by the circumferential nonelastic jacket surrounding the abdominal contents.

The jacket must be very carefully fabricated and fit for it to accomplish its mission without causing additional skin problems. Air-permeable fabric is a good material choice for the jacket because it is both flexible enough to accommodate activity without abrasion, and it permits air flow. Incidentally, a well-made abdominal jacket also may improve pulmonary function and reduce the cerebral hypotension problems experienced by some people with spinal cord injuries (21) as well as reduce back pain associated with kyphotic collapse.

The mechanics of how spine alignment affects pressure management were discussed earlier. Additionally, pelvic orientation and what is done to control it helps limit kyphoscoliotic spine collapse. The pelvis is the foundation the spine is firmly built into, via the sacrum. The biomechanics of how the seat shell and the abdominal jacket each contribute to spine stability have been thoroughly described (19,22). See Figures11a,b , c,d demonstrate the postural changes associated with the pelvic and spine support provided by the seat-and-jacket combination just described.

Interface Material Considerations

All material layers interposed between the client's skin and the sitting support surface are very important. Those layers of material can be a benefit, a liability or, more likely, some combination of the two. Those layers may, for instance, promote air circulation, helping to keep the skin surface drier and cooler. They may minimize friction at one of the interfaces between the skin and the sitting support surface, which would be valuable in minimizing tissue damage due to shear.

An ideal seat cover would be loosely fitted and made of a very flexible, low-friction material that is easily stretched in all directions. This would allow areas of bony prominence to sink into the recesses of the sitting support surface without developing tension loads within the covering fabric. Those tension loads in the cover fabric, to the extent that they develop, can apply their own load to bony prominences, and for that matter, to any convex surface feature of the contacting anatomy.

The client's clothing is at least one layer of material, in addition to the seat covering, interposed between the skin and the sitting support surface. It is important to counsel and advise clients about their selection of clothing. Some clients with decubitus ulcers arrive wearing blue jeans with rivets at the corners of the back pockets. Clients with sacral ulcers have come in wearing trousers with a belt loop contributing to the pressure against the ulcer.

We counsel people to wear no underwear, especially not jockey shorts. We advise that they remove all rear trouser pockets and belt loops. We advise them, within the limits of fashion and their cosmetic concerns, to wear the same types of materials that would work best as a cushion cover. That is, material that is thin, flexible and stretchable.

Conclusion

When people go home after rehabilitation, they function within a fairly narrow margin of safety. As the years pass, that margin of safe function gradually narrows. For many of these people, a traumatizing incident or their inability to operate within their limited margin of safety may cause a skin breakdown. If that is a full-thickness breakdown, even a complete healing process will leave skin that is at elevated risk for damage in the future.

Skin breakdowns over the isehial tuberosities or sacrum, however, are not necessarily caused by sitting conditions. In many cases, ulcers over the greater trochanters or the sacrum can be traced back to bedtime hours. Some ischial ulcers, when fully investigated, clearly are due to daily incidents such as sliding across bed sheets or across transfer boards while dressing. The tissue damage potential of such shearing incidents should never be underestimated. In other cases, rather regular traumatic injuries may be incurred as people drop down hard against a bathtub rim or toilet seat, or scrape against some hardware during transfers into and out of a wheelchair.

The use of custom orthotic seating, in conjunction with a fabric abdominal jacket, effectively reduces the risk of decubitus ulcer formation and can promote healing of existing ulcers due to sitting conditions. Helping people to heal or avoid decubitus ulcers over the long term, however, requires more than effective technology. It also requires a program of education, monitoring and follow-up (13).

Clients must be educated about what causes decubitus ulcers and how to prevent them. They need to know how to move safely across a bed, or along a sliding board, and how they can transfer safely from the chair to a car seat or into a bathtub. They need to know the relationship between clothing and pressure sore risk. They need good equipment, and that equipment should be inspected at least annually so needed repair and refurbishing can occur before any skin breakdown occurs.

Decubitus ulcers, especially among people with spinal cord injuries, bear a huge price tag. In the five years after a spinal cord injury, almost 30 percent of that population develop a pressure ulcer; one-fifth of those sores are severe (1). The consequences may be disrupted lives, amputated limbs and even death. The cost to society in terms of medical care dollars is staggering. People with a severe pelvic sore can expect to be hospitalized an average of two months for that sore. Their hospital charges are three to four times greater than for people without sores (1). Custom-sitting support orthotics is a specialty service that can help bring solutions to this prevalent and costly medical problem.

J. MARTIN CARLSON, MS, CPO, is founder and president of Tamarack Habilitation Technologies Inc., 1471 Energy Park Drive, St. Paul, MN 55108.

MARK J. PAYETTE is an orthotist and seating/adaptive equipment specialist at Tamarack Habilitation Technologies Inc.

LISA P VERVENA, MS, is a rehabilitation engineer and the research project and grant coordinator at Tamarack.

References:

1. Young JS, Burns PB, Bowen AM, McCutchen R. Spinal cord injury statistics: experience of the regional spinal cord injury systems. Phoenix: Good Samaritan Medical Center, August 1982.
2. Kosiak M. Etiology of decubitus ulcers. Arch Phys Med Rehab January 1961 ;42:19-29.
3. Roaf R. The causation and prevention of bed sores. In: Kenedi RM, Cowden JM, Scales JT [eds]. Bed sore biomechanics. London and Basingstoke: The MacMillan Press Ltd. 1976:5-9.
4. Bennett L, Lee B. Pressure and shear in pressure sores. Proceedings of the National Symposium on the Care, Treatment and Prevention of Decubitus Ulcers. Arlington, Va. November 1984:41-6.
5. Bennett L, Kavner D, Lee B, Trainor F, Lewis J. Skin stress and blood flow in sitting paraplegic patients. Arch Phys Med Rehab April 1984;65:186-90.
6. Houle RJ. Evaluation of seat devices designed to prevent ischemic ulcers in paraplegic patients. Arch Phys Med Rehab October 1969; 50:10:587-94.
7. Landis EM. Micro-injection studies of capillary blood pressure in human skin. Heart. May 1930;15:209.
8. Guyton AC. Textbook of medical physiology. Philadelphia: WB Saunders Co., 1981:363-4.
9. Lindan 0, Greenway R, Piazza J. Pressure distribution on the surface of the human body: I. Evaluation in lying and sitting positions using a "bed of springs and nails." Arch Phys Med Rehab May 1 965;46:378-85.
10. Letts RM. Pelvic obliquity and seating. In: Letts RM [ed]. Principles of seating the disabled. Boca Raton: CRC Press Inc., 1991:131-50.
11. Garber SL, Krouskop T. Body build and its relationship to pressure distribution in the seated wheelchair patient. Arch Phys Med Rehab January 1 982;63:17-20.
12. Kosiak M. Etiology and pathology of ischemic ulcers. Arch Phys Med Rehab February 1 959;40:62-9.
13. . Kosiak M. Prevention and rehabilitation of pressure ulcers. Decubitus May 1991;4:2:60-8.
14. Mahanty 5, Roemer R. Thermal response of skin to application of localized pressure. Arch Phys Med Rehab December I 979;60:584-90.
15. Bennett L, Kavner D, Lee B,Trainor F. Shear vs pressure as causative factors in skin blood flow occlusion. Arch Phys Med Rehab I 979;60:309-1 4.
16. Dinsdale S. Decubitus ulcers: role of pressure and friction in causation. Arch Phys Med Rehab April 1974;55:147-52.
17. Ferguson-Pell M, Reddy NP, Stewart SFC, Palmieri V, Cochran GVB. Measurement of physical parameters at the patientsupport interface. In: Whittle M, Harris D [edsj. Biomechanical measurement in orthopaedic practice. Oxford: Clarendon Press, 1985:133-44.
18. Grundy D, Swain A, Russell J. ABCs of spinal cord injury: medical management in the spinal injuries unit. British Med J January 18, 1986;292:183-7.
19. Carlson JM, Lonstein J, Beck KO, Wilkie DC. Seating for children and young adults with cerebral palsy. Clin Pros and Orth 1987;11:3:176-98.
20. Carlson JM, Winter R. The "Gillette" sitting support orthosis. Orth and Pros December 1978;32:4:35-45.
21. McCool ED, Pichurko BM. Slutsky AS, Sakarati M, Rossier A, Brown R. Changes in lung volume and rib cage configuration with abdominal binding in quadriplegia. J Appl Physiol 1986;60:4:1198-202.
22. Morris JM, Lucas DB, Bresler B. Role of the trunk in stability of the spine. JBJS April 1961;43-A:3:327-51.